We have investigated nonalloyed Al/Pt ohmic contacts on n-type ZnO:Al (n d ϭ2.0ϫ10 18 cm Ϫ3 ). It is shown that the as-deposited Al/Pt contacts produce a specific contact resistivity of 1.2 ϫ10 Ϫ5 ⍀ cm 2 . Auger electron spectroscopy and x-ray photoelectron spectroscopy depth profile results show interdiffusion between oxygen and aluminum, resulting in an increase of carrier concentrations near the ZnO surface. The increase of the carrier concentration at the surface region of ZnO is attributed

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... to the low resistance of the nonalloyed Al/Pt contact. Zinc oxide ͑ZnO͒ is considered to be a promising II-VI compound semiconductor for applications in optical devices such as blue light emitting diodes ͑LEDs͒, laser diodes ͑LDs͒, and ultraviolet ͑UV͒ photodetector, because of its wide direct band gap (E g ϭ3.37 eV), a large exciton binding energy of 60 meV, and easy wet-chemical processing. 1,2 For the last few years, great progress in the fabrication of ZnObased LEDs, such as p-n homojuctions using excimer laser doping 3 and heterojuctions using p-SrCu 2 O 2 , has been achieved. 4 Despite such progress in ZnO-based devices, there are still some problems which remain to be solved, such as difficulty in the growth of p-ZnO and lack of high quality ohmic or Schottky contacts to ZnO. In particular, in order to realize high-performance ZnO-based optoelectronic devices, it is essential to achieve highly reliable ohmic contacts. In our previous work, we showed that Ti/Au and Ru schemes for n-ZnO produce specific contact resistivity of ϳ10 Ϫ4 Ϫϳ10 Ϫ5 ⍀ cm 2 upon annealing. 5-7 Recently, Liang et al., 8 fabricated ZnO-based UV photoconductive detectors using Al as an ohmic contact electrode. Very recently, Xiong et al., 9 investigating the electrical properties of homojunctions of n-ZnO on p-ZnO, showed that Al/Au ohmic contacts to both n-and p-ZnO exhibit linear I -V characteristics. However, detailed examinations of electrical behaviors and ohmic contact mechanisms for the Al-based contacts were not performed. We report on the formation of low-resistance nonalloyed ohmic contacts on n-ZnO:Al using Al/Pt metallization scheme. It is shown that the Al/Pt contacts yield a low specific contact resistivity of 1.2ϫ10 Ϫ5 ⍀ cm 2 without an annealing process. Possible mechanism to explain nonalloyed Al/Pt ohmic contact was suggested by using AES and XPS examinations. A specially designed high temperature epitaxy rfsputtering system ͑HTE-rf sputtering system: KVS-25060͒ was used to grow 1-m-thick Al-doped ZnO on a ͑0001͒ sapphire substrate at 900°C using a 2 in-target containing 1 wt % Al 2 O 3 powder ͑Pure Tech͒. 10 The as-grown n-ZnO layer was then rapid-thermal annealed at 900°C for 3 min in nitrogen ambient in order to improve the crystallinity and activate the Al dopants. The carrier concentration (2.0 ϫ10 18 cm Ϫ3 ) and the mobility (60 cm 2 /V s) of the annealed layer were measured at room temperature by means of Hall effect measurement with the Van der Pauw geometry. Prior to lithography, the samples were ultrasonically degreased with trichloroethylene, acetone, and methanol for 1 min in each step, and then rinsed with DI water. Specific contact resistivity was determined using a circular-transmission line method (c-TLM͒ which obviates the need for the fabrication of mesa structures by implantation or etching processes. 11 c-TLM patterns were formed by standard photolithography and a liftoff technique. The inner radius of the patterns was 105 m and the spacings between the inner and outer radii ranged from 3 to 21 m. Prior to metal deposition, the c-TLM-patterned layer was etched by treatment with a buffered oxide etch ͑BOE͒ solution for 30 s to remove any layers of contamination and was then blown dry with N 2 . Subsequently, the Al ͑60 nm͒ and Al ͑20 nm͒/Pt ͑50 nm͒ films were then deposited on the n-ZnO by electron beam evaporation ͑PLS 500 model͒. Current-voltage (I -V) data were measured using a parameter analyzer ͑HP 4155A͒. To characterize the extent of indiffusion between Al and n-ZnO, Auger electron spectroscopy ͑AES͒ ͑PHI 670 model͒ with electron beam of 10 keV and 0.0236 A and x-ray photoelectron spectroscopy ͑XPS͒ depth profiling ͑a PHI5700 ESCA system͒ with an Al K␣ line ͑1486.6 eV͒ were performed. The interfacial reaction products were identified by glancing angle x-ray diffraction ͑GXRD͒, which was carried out using a Rigaku diffractometer ͑D/MAX-RC͒. a͒ Electronic